1. Field of Invention
This invention relates to vehicle tires, or other rolling members where it is desirable to monitor the coefficient of friction within the contact area between the rolling member and the rolling surface, and have a skid warning indicator.
2. Description of Prior Art
Since the arrival of automotive anti-skid brake systems and automatic traction control systems, it has become increasingly desirable to monitor the coefficient of friction between a vehicle's tires and the roadway in order to increase vehicle safety and fuel efficiency. The availability of powerful, sturdy computers and processors for automotive applications opens the door for sophisticated automated vehicle performance monitoring and control systems.
Unfortunately, existing methods do not adequately address the problems associated with monitoring the coefficient of friction in the footprint, and do not provide adequate information to the vehicle operator. Specific problems with existing methods include:
(a) measurement of the coefficient of friction requires multiple tires in contact with a roadway; thus continuous knowledge of conditions under individual tires cannot be determined PA1 (b) they rely on slip of the entire footprint area PA1 (c) the methods are insensitive when the maneuvering forces within the footprint are low PA1 (d) they require extensive data processing, and data from multiple types of sensors to remove noise and extract useful coefficient of friction information PA1 (e) they require extensive modifications to tire structure and materials to accommodate the tread force sensors PA1 (f) they cannot be installed after the tire manufacture is completed PA1 (g) they do not provide the vehicle operator with a qualitative indication of the roadway coefficient of friction, nor do they provide a skid warning indication PA1 (h) they do not provide an indication of the tread wear and overall tire tread condition, nor do they provide a wearout indication. PA1 (a) to provide a method and system to determine the coefficient of friction within the footprint of a single tire in contact with a rolling surface, at least once each revolution, by monitoring only a discrete element of the tread within the footprint such that the forces present on that discrete element of tread are sensed as the discrete element passes through the footprint. Slipping of the tread is determined by a change in the forces on the discrete tread elements that are not associated solely with the discrete element entering or leaving the footprint. PA1 (b) to provide a method to detect the slippage of a discrete element of the tread within the footprint of a rolling tire, that does not require slippage of all tread within the footprint. In other words, slippage of the entire footprint area, and a resultant change in wheel rotational speed is not required to determine the coefficient of friction in the footprint. PA1 (c) to provide a method of monitoring the forces on a discrete element of tread within the footprint of a rolling tire sensitive enough to detect the slippage of that discrete element of tread under very light maneuvering loads, or by sensing the gripping forces of the pneumatic tire under approximately zero maneuvering loads. PA1 (d) to provide a method of detecting the forces on a discrete element of tread within the footprint of a rolling tire while isolating the sensor from stresses and forces in the tire structure; thus minimizing the ability of various noise sources to couple into the sensor, reducing the processing required to extract meaningful coefficient of friction information, and eliminating the need for information from other sensors (such as accelerometers, tire temperature sensors, and tire pressure sensors). PA1 (e) to provide a method of detecting the forces on a discrete element of tread within the footprint of a rolling tire by means of a sensor assembly that can be incorporated into a conventional tire without extensive redesign of the tire structure, or the tread materials. PA1 (f) to provide a method of detecting the forces on a discrete element of tread within the footprint of a rolling tire by use of a sensor assembly that can be installed in a tire after the tire manufacture is completed. PA1 (g) to provide a system whereby the coefficient of friction is determined from the forces sensed in a discrete element of tread within the footprint of a rolling tire, such that the coefficient of friction is presented to the vehicle operator of the vehicle in the form of a qualitative assessment of roadway coefficient of friction as well as a skid warning indication. PA1 (h) to provide a tire wear out indication and overall assessment of tread condition.
Anti-skid braking systems, such as those described by Okazaki in U.S. Pat. No. 5,466,054 (14 Nov. 1995), by Walenty et al in U.S. Pat. No. 4,916,619 (10 Apr. 1990), by Rath in U.S. Pat. No. 4,779,447 (25 Oct. 1988), and by Leiber in U.S. Pat. No. 4,545,240 (8 Oct. 85) determine the coefficient of friction by monitoring the rotation of two or more wheels during braking or acceleration. A difference in wheel rotation is interpreted as slip. The slip information is combined with brake pressures and other parameters to derive an estimate of the coefficient of friction for the slipping wheel. For these systems to provide coefficient of friction information, slippage must occur everywhere within the footprint, and result in a significant change in tire rotational speed. During the slip, the affected wheel is not able to effectively control vehicle motion; therefore vehicle maneuvering forces must be reacted by the other tires. The added load on the remaining tires increases the potential for skid. Furthermore, it is not always obvious which of the two or more tires is slipping since the system is based on a difference in wheel speeds. In addition, these systems do not take the extra step to create and display an indication of coefficient of friction, or skid warning for the vehicle operator.
Other anti-skid systems, such as those described by Kuwana et al in U.S. Pat. No. 5,236,255 (17 Aug. 1993) and by Kuwana et al in U.S. Pat. No. 5,056,354 (15 Oct. 1991) rely upon acceleration sensors to detect wheel slip and have the same disadvantages discussed above.
Methods that measure the forces exerted by the wheel at the tire footprint have been described by Fioravanti in U.S. Pat. No. 5,247,831 (28 Sep. 93) and in European patent EP-A-0 233 357. Fioravanti's method requires extensive modifications to conventional tire structure, materials and construction methods to allow the incorporation of flexible toroidal strain sensors within the tire structure. They cannot be installed in a tire after it has been built since the sensors are an integral part of the tire structure. These modifications will force tire manufacturers to change their processes extensively and will adversely affect tire performance because Fioravanti's method requires the use of extensometric transducers constructed of piezoresistive rubbers within the tread. Such materials do not have suitable wear characteristics for an automotive tire, and the materials are not readily blended or integrated with existing tire tread manufacturing materials and manufacturing techniques. The fact that tires such as those described by Fioravanti do not exist on the commercial market today, is evidence of the manufacturing problems inherent in the design. There are also significant operational problems with Fioravanti's design. The sensors are excessively sensitive to tire tread forces resulting from maneuvering loads, tire pressure changes, centrifugal forces and tire tread deformation within the footprint. The sensitivity results, in part, from the coupling of tire structure forces into the tread sensors. For example, as the tire pressure, or rotational speed changes, the tire structure will expand, forcing the tread to expand as well. This expansion will influence the tread sensor output and will be indistinguishable from tread forces caused by maneuvering loads without additional sensor information and signal processing. This is to be expected since the sensors described by Fioravanti were intended primarily to determine the full range of forces acting on the tire tread within the footprint. Fioravanti's sensors are not optimized for the determination of the coefficient of friction in the footprint. Additional sensors to measure tire pressure, tire temperature and vehicle acceleration will be required to extract useful coefficient of friction information, and due to the number, and magnitude, of forces acting on the tread sensors, it requires significant processing to extract coefficient of friction information and eliminate noise.
European patent EP-A-0 233 357 documents a method wherein sensors are connected between the wheel rim (or tire bead) and the inside of the tire. This methods has drawbacks similar to Fioravanti's; however, it has the added problem of being coupled entirely to the tire structure, (specifically the inside of the pneumatic tire such as indicated by 16 of FIG. 1), rather than the tread itself. This will make it unsuitable for sensing tire tread slip under light maneuvering loads. Like those already discussed, these systems do not take the extra step to create and display an indication of coefficient of friction, or a skid warning for the vehicle operator.
None of the known methods are designed to take advantage of the tire gripping forces that are a natural result of a pneumatic, or otherwise elastic, tire that is deforming under a load as it rolls on the roadway.
Objects and Advantages
Accordingly, several objects and advantages of our invention are:
Further objects and advantages are: to provide improved roadway coefficient of friction information to existing anti-skid braking systems, automatic traction control systems and other systems designed to improve the handling of a vehicle or which would benefit from roadway coefficient of friction information, and to provide the ability to sense slippage in lightly loaded wheels, tires and rollers such as those used in the paper milling and copier industries to feed and guide paper.
Further objects and advantages of our invention will become apparent from consideration of the drawings and ensuing description.